5. Save extra information about the drive geometry necessary in order to interpret the partition table stored within the image. The most important of which is the cylinder size.

5. Save extra information about the drive geometry necessary in order to interpret the partition table stored within the image. The most important of which is the cylinder size.

Revision as of 02:20, 5 November 2015

zh-CN:Disk cloning
Disk cloning is the process of making an image of a partition or of an entire hard drive. This can be useful for copying the drive to other computers and for backup and recovery purposes.

Using dd

The dd command is a simple, yet versatile and powerful tool. It can be used to copy from source to destination, block-by-block, regardless of their filesystem types or operating systems. A convenient method is to use dd from a live environment, as in a Live CD.

Warning: As with any command of this type, you should be very cautious when using it; it can destroy data. Remember the order of input file (if=) and output file (of=) and do not reverse them! Always ensure that the destination drive or partition (of=) is of equal or greater size than the source (if=).

Cloning a partition

Warning: If output file of= (sdb1 in the example) does not exist, dd will create a file with this name and will start filling up your root file system!

Cloning an entire hard disk

From physical disk /dev/sdX to physical disk /dev/sdY

# dd if=/dev/sdX of=/dev/sdY bs=512 conv=noerror,sync

This will clone the entire drive, including the MBR (and therefore bootloader), all partitions, UUIDs, and data.

noerror instructs dd to continue operation, ignoring all read errors. Default behavior for dd is to halt at any error.

sync fills input blocks with zeroes if there were any read errors, so data offsets stay in sync.

bs=512 sets the block size to 512 bytes, the "classic" block size for hard drives. If and only if your hard drives have a 4 Kib block size, you may use "4096" instead of "512". Also, please read the warning below, because there is more to this than just "block sizes" -it also influences how read errors propagate.

The dd utility technically has an "input block size" (IBS) and an "output block size" (OBS). When you set bs, you effectively set both IBS and OBS. Normally, if your block size is, say, 1 Mib, dd will read 1024*1024 bytes and write as many bytes. But if a read error occurs, things will go wrong. Many people seem to think that dd will "fill up read errors with zeroes" if you use the noerror,sync options, but this is not what happens. dd will, according to documentation, fill up the OBS to IBS size after completing its read, which means adding zeroes at the end of the block. This means, for a disk, that effectively the whole 1 Mib would become messed up because of a single 512 byte read error in the beginning of the read: 12ERROR89 would become 128900000 instead of 120000089.

If you are positive that your disk does not contain any errors, you could proceed using a larger block size, which will increase the speed of your copying several fold. For example, changing bs from 512 to 64 Ki changed copying speed from 35 MB/s to 120 MB/s on a simple Celeron 2.7 GHz system. But keep in mind that read errors on the source disk will end up as block errors on the destination disk, i.e. a single 512-byte read error will mess up the whole 64 Kib output block.

Tip: If you would like to view dd progressing, use the status=progress option. See dd for details.

Note:

To regain unique UUIDs of an ext2/3/4 filesystem, use tune2fs /dev/sdXY -U random on every partition.

Partition table changes from dd are not registered by the kernel. To notify of changes without rebooting, use a utility like partprobe (part of GNU Parted).

Backing up the MBR

The MBR is stored in the the first 512 bytes of the disk. It consist of 3 parts:

The first 446 bytes contain the boot loader.

The next 64 bytes contain the partition table (4 entries of 16 bytes each, one entry for each primary partition).

The last 2 bytes contain an identifier

To save the MBR as mbr.img:

# dd if=/dev/sdX of=/path/to/mbr_file.img bs=512 count=1

To restore (be careful: this could destroy your existing partition table and with it access to all data on the disk):

# dd if=/path/to/mbr_file.img of=/dev/sdX

If you only want to restore the boot loader, but not the primary partition table entries, just restore the first 446 bytes of the MBR:

5. Save extra information about the drive geometry necessary in order to interpret the partition table stored within the image. The most important of which is the cylinder size.

# fdisk -l /dev/sdX > /path/to/list_fdisk.info

Note: You may wish to use a block size (bs=) that is equal to the amount of cache on the HD you are backing up. For example, bs=8192K works for an 8 MiB cache. The 64 Kib mentioned in this article is better than the default bs=512 bytes, but it will run faster with a larger bs=.

Restore system

To restore your system:

# gunzip -c /path/to/backup.img.gz | dd of=/dev/sdX

When the image has been split, use the following instead:

# cat /path/to/backup.img.gz* | gunzip -c | dd of=/dev/sdX

Disk cloning software

Disk cloning in Arch

Partclone provides utilities to save and restore used blocks on a partition and supports ext2, ext3, ext4, hfs+, reiserfs, reiser4, btrfs, vmfs3, vmfs5, xfs, jfs, ufs, ntfs, fat(12/16/32), exfat. Optionally, an ncurses interface can be used. Partclone is available in the community repository.

Partimage, an ncurses program, is available in the community repos. Partimage does not currently support ext4 or btrfs filesystems. NTFS is experimental.

Disk cloning outside of Arch

If you wish to backup or propagate your Arch install root, you are probably better off booting into something else and clone the partition from there. Some suggestions:

PartedMagic has a very nice live cd/usb with PartImage and other recovery tools.

Mindi is a linux distribution specifically for disk clone backup. It comes with its own cloning program, Mondo Rescue.

Acronis True Image is a commercial disk cloner for Windows. It allows you to create a live (from within Windows), so you do not need a working Windows install on the actual machine to use it. After registration of the Acronis software on their website, you will be able to download a Linux-based Live CD and/or plugins for BartPE for creation of the Windows-based live CD. It can also create a WinPE Live CD based on Windows. The created ISO Live CD image by Acronis doesn't have the hybrid boot ability and cannot be written to USB storage as a raw file.